Unified model of catalyzed and uncatalyzed decomposition of ammonium perchlorate

COMBUSTION

A N D F L A M E 51: 177-182 (1983)

177

Unified Model of Catalyzed and Uncatalyzed Decomposition of Ammonium Perchlorate R. P. RASTOGI, A. P. RAO, and V. SYAL Chemistry Department, Gorakhpur University, Gorakhpur-273001, U.P. India

Catalyzed and uncatalyzed thermal decomposition of ammonium perchlorate (AP) has been investigated at different temperatures in the range 220-315°C for particles of different sizes. The data are fitted by the integrated form of the relation d~

as

ba 2

- - =

dt

1 + dc~

where a denotes the fraction decomposedat time t and a. b, and d are constants. At lowertemperaturesda < < 1, while at higher temperatures dt~ > > 1.The constantsa, b, and d depend strongly on temperature. The energy of activationof the processes associated with a and b is 45 -_+1 kcal mol- J while that for d is 42 _.+ 1 kcal mol - J.

INTRODUCTION There is considerable current interest [1-12] in the decomposition behavior o f ammonium perchlorate since it is widely used as an oxidizer in solid propellant rockets. While a lot of experimental data is available [1, 2], confusion persists regarding the correlation o f the data at different temperatures. Even the criteria of catalysis for AP decomposition has not been clearly defined. It has been the practice to treat the low-temperature and high-temperature decomposition of AP separately, and the transition in behavior had been ascribed to the polymorphic transition at 240°C. The general tendency had been to split the curve for extent o f reaction (a) against time into two intersecting straight lines and to fit them by an appropriate analytic equation, e.g., the ProutTompkin equation. Jacobs and Ng attempted to fit the data with a 10-constant equation [3], but the concept is not essentially different. The point o f view which is presented in this paper is that the Copyright t~) 1983 by The Combustion Institute Published by Elsevier Science PublishingCo., Inc. 52 Vanderbilt Avenue, New York, NY 10017

basic chemical processes in the two regimes are the same, and hence a single analytical equation involving fewer constants should describe the entire range o f data. Thus, in the present work, data on AP decomposition at (i) different temperatures (ii) for particles o f different sizes and (iii) for different additives has been obtained. These data do not supersede those of earlier workers, nor are they more accurate, but the present results are for the same specimen of AP, so that they are directly comparable.

Experimental

(i) Materials and Preparation of Samples The AP (ISRO, Alwaye, Kerala) had the following specification. Assay = 99.00%, ash = 0.25%, chloride = 0.10%, chlorate = 0.10%, H 2 0 insolubles = 0.10%, moisture = 0.08%. Before use, the crystalline AP was ground and sieved through 100-200,

O010-2180/83/$03.00

178

R. P. RASTOGI ET AL.

200-300, 300-400, mesh sieves. Manganese carbonate (BDH India) and basic copper carbonate (BDH India) were used without further purification. Copper chromite was prepared in the laboratory by heating basic copper carbonate and chromium carbonate in the ratio 1 : 2 . 3 7 by weight at 800°C for 2 h. (ii) Isothermal TGA of AP (100-200 Mesh) at Various Temperatures Samples of AP were dried beforehand in a desicator. The studies were made with a locally fabricated TGA apparatus in the temperature range 220-315°C. The temperature of the furnace was controlled to -+I°C and studies were made using ~100 mg of AP.

RESULTS AND DISCUSSION It is suggested that the following 3-constant equation applies over the entire temperature range for the decomposition of AP: da

a a - - bcL2

dt

1 + da

(1)

where a is fraction of AP decomposed at time t and a, b, and d are constants. Two extreme cases of this equation are

(i) dc~ '~ 1 when Eq. 1) reduces to

- aa -- ba 2 ;

(2)

dt

(iii) Reproducibility of the Results The estimated error in t~ arising from the uncertainty in the time ranges from 3 to 10%, the latter value corresponding to the region near the point of inflexion, but the results of two runs plotted in Fig. 1 show that the reproducibility is better than this. The initial mass of the AP sample only varied by -+1%; hence the values of the constants in Eqs. (2) and (3) are correct to -+2-3%.

(ii) da >> 1 when da --=a dt

I

I

-b

a,

(3)

b' = b i d .

(4)

where a' = a / d ,

0-8

~0

0.6

T

0

0.4

0.?.

IOO

t (mi.~ P

400

5~

Fig. 1. R e p r o d u c i b i l i t y o f t h e r m a l d e c o m p o s i t i o n m e a s u r e m e n t s A P ( 1 0 0 - 2 0 0 m e s h ) + M n C O a 4 % ( 1 0 0 - 2 0 0 m e s h ) ; t e m p e r a t u r e = 2 4 5 ° -+ I ° C ; o ~ I r u n ; • ~ II r u n .

DECOMPOSITION OF AMMONIUM PERCHLORATE

179

0.3

0.2

0.1

0

I00

Fig. 2. T e s t o f E q . (2); A P ( 1 0 0 - 2 0 0 mental; • = theoretical.

ZOO t train)

460

to satisfy the integrated form of Eq. (3), viz.,

a

--a l n a _ b a

400

m e s h ) ; t e m p e r a t u r e = 2 2 0 ° _+ I ° C ; o ~ e x p e r i -

Integration of Eq. (2) yields 1

300

1

-t+C,

(5)

where C is an integration constant. Rearrangement gives

----b' l n ( a ' - - b ' a ) - - t + C '

where C' is another constant. Equation (6) on rearrangement gives

a b + e -K" where K = t + C. Equation (5) cannot be tested directly and thus the following approach was adopted. Values of (1/a)(d~/dt) were plotted against a and the best values of slope and intercept were determined; these gave an estimate of a and b [see Eq. (2)]. Further, in order to estimate C, (l/a) In [~/(a - b a ) ] was plotted against t, according to Eq. (5). Using these values of a, b, and C, the fraction decomposed at any time t was calculated, and the theoretical curves were obtained, for comparison with the experimental values. Typical results for a temperature <245°C are given in Fig. 2, which shows that satisfactory agreement was obtained. Equations (2) and (5) do not satisfy the data for temperature >245°C. However, for much higher temperatures (-~290°C) the data was found

(6)

a =

a' -- e - ° '(t+C ') b'

(7)

The constants a' and b' were estimated from the slope and intercept of the plot of d ~ d t against a using Eq. (3), while C' was estimated from the intercept of the plot o f - ( 1 / b ' ) ln(a' - b ' a ) against t [see Eq. (6)]. Using these constants, a was calculated for different values of t, and the values obtained were found to be in satisfactory agreement with the experimental values. This is illustrated by typical plot shown in Fig. (3). In the intermediate temperature range Eq. (1) was used for the analysis of the data which was tested as follows. Integration of Eq. (1) gives -

a

In a --

+

ln(a -- bc0 = t + constant. (8)

In order to evaluate a at various t using Eq. (8),

180

R.P. RASTOGI ET AL.

0-3

T

0.2

O'l

I

!

so

,oo

Fig. 3. T e s t o f E q . (3); A P ( 1 0 0 - 2 0 0 mental; • ~ theoretical.

,~o

2~o

m e s h ) ; t e m p e r a t u r e = 3 1 5 ° _* 1 ° C ; o ~ e x p e r i -

the values of a, b, d, and the constant are needed, and these were obtained by sucessive approximation. First of all, the values of the constants in Eq. (8) was evaluated from the known values of a' and b' by assuming a suitable value of d. Values of a were then calculated using Eq. (8) and compared with experimental values. The process was then repeated using different values of d until a

reasonable agreement was obtained between the calculated and experimental values of a. Theoretical a - t curves were then drawn and compared with the experimental curves for a number of cases and a typical result is plotted in Fig. 4. In view of the uncertainties in a, b, and d, the agreement between the calculated and experimental curves in Fig. 4 is as good as can be expected.

0.8

T 0.6 0.4

0

I00

ZOO t (m;~)

300

I

400

-':-

Fig. 4. T e s t o f E q . (1); A P ( 1 0 0 - 2 0 0 m e s h ) + M n C O 3 4% ( 1 0 0 - 2 0 0 m e s h ) ; t e m p e r a t u r e = 2 4 5 ° _+ I ° C ; a = 0 . 4 3 5 5 ; b = 0 . 4 1 6 ; d = 6 5 ; ( a ) t = 0 = 0 . 0 0 2 5 ; • = t h e o r e t i c a l ; o ~ experimental.

DECOMPOSITION

OF AMMONIUM

181

PERCHLORATE TABLE 1

Values o f Constants for AP Decomposition at Different Temperatures (Particle Size 1 0 0 - 2 0 0 Mesh) Temp. (°C)

a (min--1)

a' (min--1)

b (min--1)

b' (min--1)

d

C

C'

220 230 237 245 253 265 290 315

0.0210 0.0420 0.0650 0.1290 a 0.2590 a 0.8190 a

0.0114 a 0.0119 a 0.0122 a 0.0125 0.0135 0.0145 0.0195 0.0200

0.0785 0.1660 0.2310 0.4720 a 0.9950 a -

0.0388 a 0.0402 a 0.0428 a 0.0387 0.0474 0.0550 0.0666 0.0700

1.92 3.79 5.35 11.58 20.07 -

-70 -65 -45 -

102 75 58 65

-

a Extrapolated values from plots of parameters against reciprocal of absolute temperature.

TABLE 2 Values of Constants o f Eqs. (2) and (3) for Decomposition o f AP of Various Particle Sizes at 245°C Particle size o f AP (mesh)

a (min--1)

b (min--1)

a' (min--1)

b' (min--1)

100-200 200-300 300--400 a

0.053

0.181

0.0125 0.0085 .

0.0387 0.0270 .

.

C'

C

102 160

30

.

a At 245°C, AP (300--400 mesh) has a sigmoidal curve whereas AP ( 1 0 0 - 2 0 0 mesh) has a nonsigmoidal curve.

TABLE 3 Values o f Constants of Eq. (3) for Decomposition of AP ( 1 0 0 - 2 0 0 Mesh) in Presence o f Catalysts ( 1 0 0 - 2 0 0 Mesh) at 245°C a

Catalysts Nil MnCO a (D) BCC (B) CC (B)

a' = aid (min--1)

b' = bid (min--1)

a (min--1)

b (min--1)

d

C'

0.0125 0.0067 0.0125 0.0082

0.0387 0.0064 0.0300 0.0200

0.125 0.435 0.125 0.246

0.387 0.42 0.30 0.60

10 65 10 30

102 757 112 228

a (D) denotes good decomposition catalyst; (B) denotes good burning rate catalyst.

T h e v a l u e s o f t h e p a r a m e t e r s a, b, a', b', a n d d obtained

in

the

present

study

are

recorded

in

T a b l e s 1 - 3 . It is c l e a r t h a t a/a' = b i b ' = d w i t h i n experimental sistent. constants

The

error, showing that results are contemperature

dependence

of

these

f o l l o w s a n A r r h e n i u s l a w (see Figs. 5),

and

the

corresponding

activation

energies

are

E , = E b = 4 5 . 5 k c a l / m o l , E a, = E b, = 3 . 8 k c a l / mol and E a = 42.4 kcal/mol. Equations

( 2 ) a n d ( 3 ) w e r e also f o u n d t o b e

s a t i s f a c t o r y f o r A P p a r t i c l e s o f d i f f e r e n t sizes, a n d the resultant parameters

a r e r e c o r d e d in T a b l e 2.

182

R . P . RASTOGI ET AL.

-I

-2

L -4 160

170

180 I/TeK

XIO 5

190 ,

200

Fig. 5. Plot of In X against the reciprocal of absolute temperature (X = a, b, a', b'); X=o~a;X=o*b:X=A~a';X=zx=b '.

In order to examine the validity of the above equations for the case o f catalyzed decomposition o f AP, experiments with MnCO3, basic copper carbonate (BCC), and copper chromite (CC) were performed at 245°C. The latter two are known to be burning rate catalysts for composite propellant. The data are fitted by Eq. (3) and the necessary constants are listed in Table (3). The results show that the value of a is considerably enhanced by MnCO a and copper chromite but is unaffected by BCC. Further work is needed to explain the difference in behavior. Thanks are due to Dr. R. R. Singh f o r help in computation. The authors are grateful to the Department o f Science and Technology, N e w Delhi, f o r supporting the investigation.

REFERENCES

1. Jacobs, P. W. M., and Whitehead, H. M., Chem. Rev. 69:551 (1969).

2. Solymosi, F., Structure and Stability o f Salts o f Halogen Oxyacids in the Solid Phase, John Wiley, New York, 1977, Chapter 4. 3. Jacobs, P. W. M., and Ng, Wee Larn, in Reactivity of Solids." Proceedings of the Seventh International Symposium on Reactivity o f Solids (J. S. Anderson, Ed.), Chapman and Hall Ltd., London, pp. 398--410. 4. Rastogi, R. P., Singh, Gurdip, and Singh, R. R., Combust. Flame 30:117-124 (1977). 5. Rastogi, R. P., Singh, Gurdip, and Singh, R. R.,lnd. J. Chem. 15A:845 (1977). 6. Boggs, T. L., Zuru, D. E., and Cordes, H. F., Naval Weapons Center, Chinalake, California, AIAA Paper 75-233, Jan. 20-21, 1975. 7. Rastogi, R. P., Singh, Gurdip, and Singh, R. R., Combust. Flame 33:305-310 (1978). 8. Rastogi, R. P., Dubey, B. L., Singh, Gurdip, and Shukla, C. S., Journal o f Catalysis 65:25-30 (1980). 9. Kraeutle, K. J.,J. Phys. Chem. 74:1350 (1970). 10. Boldlyrev, V. V., Alexandrov, V. V., Boldyrev, A. V., Gritsan, V. I., Karpenko, Yu. Ya., Koroboeinitchev, O. P., Panfilov, V. N., and Khariretdinov, E. F., Combust. Flame 15:71-78 (1970). 11. Jacobs, P. W. M., and Pearson, G. S., Combust. Flame 13:419 (1969). 12. Galway, A. K., and Jacobs, P. W. M., J. Chem. Soc. 837-844 (1959). Received 21 January 1981; revised I December 1982